The present invention relates to a method for the synthesis of ethyl 2-[4'-(6"-chloro-2"-benzoxazolyloxy)phenoxy]propanoate. This compound is useful as a herbicide.
It is known in the art to produce herbicidal agents which are heterocyclic substituted 2-aryloxyphenoxy)alkanoic acids. Within this context aryl includes benzthiazolyl, benzoxazolyl, benzpyrozolyl, etc., derivatives. Such may be described by the generalized formula: ##STR1## in which (R).sub.n is hydrogen, halogen, CF.sub.3, NO.sub.2, CN, alkyl, alkoxy or alkylthio,
A is O, S, NH, or N-alkyl
R' is hydrogen or alkyl and
Z is a carboxylic acid, carboxylic ester, thiol ester, carbamide, carbohydrazide, thioamide, nitrile, hydroxymethyl, acyloxymethyl, carbamoylmethyl, or a sulfonyloxymethyl group. The most preferred compound is ethyl 2-[4'-(6"-chloro-2"-benzoxazolyloxy)phenoxy]propanoate and has the structure: ##STR2## The compounds of the formula I possess an asymmetric carbon center and therefore occur as pure enantiomers or as a mixture thereof. These and other compounds are more fully described in U.S. Pat. Nos. 4,589,908; 4,130,413; and 4,564,682; and German Patents DE 3502266, 3430215, 3418168, 3311285, 3236730, and 3036075, and European Patent Applications 148119 and 157225, all of which are incorporated herein by reference. One method for the production of these herbicides uses an intermediate which is a 2-(4-hydroxyphenoxy)alkanoic acid ester of the formula: ##STR3## wherein R.sup.1 is H, phenyl or C.sub.1 to C.sub.18 alkyl, preferably C.sub.1 to C.sub.4 alkyl; and
wherein R.sub.2 is C.sub.1 to C.sub.8 alkyl, preferably C.sub.1 to C.sub.4 alkyl or aryl such as phenyl or naphthyl which may be substituted or unsubstituted; and wherein A, B, C and D are independently H, S, O, N, X, C.sub.1 to C.sub.18 alkyl, or C.sub.6 to C.sub.10 aryl; and X is F, Cl, Br, I or a sulfonic ester.
Prior art processes for producing these intermediate compounds have employed hydroquinone and other compounds as starting materials. Such processes are discussed at length in U.S. Pat. Nos. 3,600,437; 4,532,346; 4,547,583; and British Patent 1,591,063. U.S. Pat. No. 4,665,212 teaches condensed hydroquinone or hydroquinone salts with certain aromatic sulfonyl containing acids, esters and salts. U.S. Pat. 4,511,731 teaches the preparation of certain propanoate monoethers of hydroquinone via sequential alkylation and oxidation of hydroxystyrene. While such processes are effective for producing herbicide precursors, they are economically disadvantageous since the selectivity and rate of conversion, and hence the yield, is relatively low; on the order of about 10%. U.S. Pat. No. 4,528,394 describes a method which improves upon this yield by using a benzaldehyde precursor such that the yield is increased to about 50%. However, this system is disadvantageous because of the vigorous reaction conditions required and undesired side reactions which occur such as the self-condensation of the benzaldehyde. These may also undergo undesired oxidation to carboxylic acids under Baeyer-Villiger conditions. The present invention improves on these methods by preparing herbicides using intermediates derived from certain ketones and conducting a Baeyer-Villiger oxidation thereon. The intermediates are prepared in a stepwise fashion and several advantages are thereby noted. These include a higher yield, perhaps in the 80-95% range, easier purification of the intermediates and less vigorous reaction conditions. The invention therefore provides a process for obtaining the foregoing herbicidal compound from a hydroxyaromatic ketone and 2,6-dichlorobenzoxazole.